The present specification relates generally to the field of integrated circuits and to methods of manufacturing integrated circuits. More particularly, the present specification relates to both a binary and an attenuating phase-shifting mask for use at multiple wavelengths.
Semiconductor devices or integrated circuits (ICs) can include millions of devices, such as, transistors. Ultra-large scale integrated (ULSI) circuits can include complementary metal oxide semiconductor (CMOS) field effect transistors (FET). Despite the ability of conventional systems and processes to put millions of.devices on an IC, there is still a need to decrease the size of IC device features, and, thus, increase the number of devices on an IC.
One limitation to the smallness of IC critical dimensions is lithography. In general, projection lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern.
Exposure of the coating through a transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) or deprotected areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation.
One alternative to optical projection lithography is EUV lithography. EUV lithography reduces feature size of circuit elements by lithographically imaging them with radiation of a shorter wavelength. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a, extreme ultraviolet (EUV)), wavelength range of lambda=50 to 700 angstroms are used in an effort to achieve smaller desired feature sizes.
In EUV lithography, EUV radiation can be projected onto a resonant-reflective reticle. The resonant-reflective reticle reflects a substantial portion of the EUV radiation which carries an IC pattern formed on the reticle to an all resonant-reflective imaging system (e.g., series of high precision mirrors). A demagnified image of the reticle pattern is projected onto a resist coated wafer. The entire reticle pattern is exposed onto the wafer by synchronously scanning the mask and the wafer (i.e., a step-and-scan exposure).
Phase-shifting mask technology has been used to improve the resolution and depth of focus of the photolithographic process. Phase-shifting mask technology refers to a photolithographic mask which selectively alters the phase of the light passing through certain areas of the mask to improve resolution and depth of focus according to principles of destructive interference. For example, in a simple attenuating phase shifting mask, a layer of material is selectively located to attenuate light passing through it and shift the light 180 degrees out of phase from light passing through adjacent areas not covered by the phase shifting material. This 180 degree phase difference causes any light overlapping from two adjacent apertures to interfere destructively, thereby reducing the width of the feature at the wafer. An attenuating phase shifting mask differs from an alternating phase shifting mask in that the alternating phase shifting mask generally does not have a partially transmitting phase shifting material, but rather includes trenches in the mask to shift the phase of transmitted light adjacent to the features.
An exemplary mask 10 is illustrated in FIG. 1. Mask 10 can be either a binary mask with an anti-reflect layer over chrome or an attenuating phase-shifting mask. If used as an attenuating phase-shifting mask, the mask 10 includes a transparent mask blank layer 12 and a shifting material layer 14. Shifting material layer 14 provides a printed circuit pattern and selectively attenuates the transmission of light from transparent layer 12 to a layer of resist on a semiconductor wafer. The light transmitted through shifting material layer 14 is attenuated and phase-shifted 180 degrees from the transmission of light through clear portions of phase-shifting mask, such as portions 18. As the light travels between phase-shifting mask 10 and the resist layer of a semiconductor wafer below (not shown), the light attenuated from phase-shifting mask 10 by shifting material layer 14 interferes constructively with the light transmitted through phase-shifting mask 10 at portions 18, to provide improved resolution and depth of focus.
Alternatively, the mask 10 can be a binary mask where layer 14 is an anti-reflective layer disposed over opaque chrome material. The clear areas 18 are designed to transmit light at highest intensity and the opaque areas 14 are designed to block the light completely. While this does not have the highest resolution, it is an example of mask 10 can be constructed.
As mentioned, various different wavelengths of light are used in different photolithographic processes. The optimal wavelength of light is based on many factors, such as the composition of the resist, the desired critical dimension (CD) of the integrated.circuit, the type of lithographic equipment, etc. Often, the optimal wavelength of light must be determined by performing a lithography test with photolithographic equipment having different wavelengths. When binary or phase-shifting masks are utilized at different wavelengths, two different masks must be fabricated for the given type with each mask being suitable for phase shifting or binary transmission of light of the desired wavelength. The fabrication of phase-shifting and binary test masks is costly. Further, comparison of the effect of the two different wavelengths printing processes is difficult. Having a test mask that is suitable for multiple wavelengths is of great utility in qualifying processes at different avelengths and makes the masks useable for multiple design rule (CD) enerations at the different wavelengths.
Thus, there is a need to pattern IC devices using non-onventional lithographic techniques. Further, there is a need to form maller feature sizes, such as, smaller gates. Yet further, there is a need to have either binary or attenuating phase-shifting masks useful at multiple wavelengths.
An exemplary embodiment is related to a method of using a dual layer feature on a mask in an integrated circuit fabrication process to provide for use of the mask at multiple wavelengths. This method can include providing a dual layer feature over a mask, where the dual layer feature is configured with layers of selected thicknesses which allow the mask to be used at multiple wavelengths; and subjecting the dual layer feature and the mask to a beam at one of the multiple wavelengths at a time. This dual layer feature can be an attenuating phase-shifting layer for an attenuating phase-shifting mask or it can be a dual layer anti-reflective coating over an additional opaque layer, such as, chrome forming a binary mask.
Another exemplary embodiment is related to a method of testing a lithographic mask design using a mask configured for use with multiple light or radiation beams of different wavelengths. This method can include providing at least two layers of material over a mask, removing a portion of the at least two layers of material to form a feature, and providing a beam at a wavelength to the mask and the at least two layers of material. The at least two layers of material have thicknesses selected to allow the mask to be used with multiple wavelengths.
Another embodiment is related to either an attenuating phase shift mask or a binary mask for use with multiple wavelengths. The mask can include a blank mask layer and a dual layer system disposed over the blank mask. The dual layer system forms segments of an anti-reflective layer over chrome (binary) or an attenuating phase-shifting material (att. -PSM) selectively placed over the blank mask layer.
Other principle features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.